Mail Queue Question

I was just curious about what causes an email to get stuck in the Queue. The most common one I see is due to a full mailbox. But others just seem random.

Today, I've had two emails with multiple recipients get stuck in the Queue. One went through went I forced it (why didn't it go through by itself?). The other won't go through, appearantly, because of one bad email address (host lookup did not complete). Why won't it send the rest of the recipients through?

There is huge number of possible reasons for email to stay in the queue, exact reason for specific email can be found in the /var/log/exim_mainlog - exim reports all activity into this log file, including activity related to undeliverable emails.

Exim should deliver every multiple recipients email to as many recipients as it can, you can check queue status using the "exim -bp" command - addresses marked with letter "D" are recipients who already got your email, addresses w/o "D" mark are recipients with failed delivery.